1. Prior Art
The particles heretofore used as tracer particles in optical flow measurements are porous particles made of SiO2, TiO2, SiC or the like which are obtainable by a coprecipitation process or from a natural material such as the mineral ore. These particles generally have a mean particle diameter of about 0.5 to 150 μm.
In a measurement of the flow velocity using a laser device such as a laser Doppler velocimeter, a phase Doppler velocimeter or the like, tracer particles somewhere between 0.5 and 10 μm in mean diameter, in particular, have so far been employed.
In technologies involving a visualization of a flowing fluid by photographing the distribution of tracer particles in the fluid with the aid of an instantaneous, powerful light source, such as a flash-light or a pulse laser, and a determination of the flow pattern from the resulting picture, particles somewhere between about 5 μm and about 150 μm in mean diameter are generally employed.
Electron microphotographs of the representative tracer particles which are conventionally employed are presented in FIGS. 3 through 14; viz. white carbon in FIGS. 3 and 4, TiO2 in FIGS. 5 and 6, talc in FIGS. 7 and 8, TiO2-talc in FIGS. 9 and 10, particles from kanto loam, and white alumina in FIGS. 13 and 14.
However, as apparent from these microphotographs, the conventional tracer particles have the following drawbacks, 1) through 5), which amplify the measurement error.    1) Because the tracer particles are morphologically not uniform, the sectional area of scattered light to be detected varies according to the real-time orientation of each particle.    2) Because the particle size distribution is broad and the sectional area of light scattering varies with different individual particles, the comparatively large particles scatter light in two or more fringe at a time.    3) Because the apparent specific gravity of the particulate tracer differs markedly from that of the fluid to be measured, the particles do not faithfully follow the on-going flow of the fluid.    4) Because the particle size distribution is broad and the apparent specific gravity also has a distribution, the particles follow the fluid flow with varying efficiencies to prevent accurate quantitation of the flow measurement.    5) Because the surface of the particle is irregular, the individual particles tend to be concatenated with each other to increase the effective particle size.
The technique used generally for launching tracer particles into a fluid comprises either extruding tracer particles from a screw feeder and driving them into the body of the fluid with the aid of an air current or suspending tracer particles in a solvent and ejecting the suspension in a mist form using an ultra-sonic humidifier. In any of the above methods, the rate of feed of the tracer particles is not constant so that the accuracy of flow measurement is inevitably sacrificed.